Optical fibers are used to carry high speed data as optical signals. These optical signals must be switched between fibers to allow a signal routing function. There are a currently number of methods for achieving switching.
FIG. 1 shows an electromechanical arrangement, where a signal in an optical fiber A is routed to fiber B by mechanically aligning fiber A with fiber B. This arrangement is bulky and mostly suited only 1.times.N configurations.
An alternative solution is to use a hybrid optical switch. This arrangement is shown in FIG. 2. Firstly, the optical signals are converted to electrical signals, which are switched in a conventional manner, and the resulting outputs of the switch matrix are converted back to optical signals. The conversion process inherently makes this type of switching unidirectional.
Semiconductor optical switches are known. Presently, these are wave guide based and rely on the change of refractive indices in the waveguides under the influence of an external electric field, current or other perturbations. The most common types are: directional coupler switches, mach-zehnder interferometers, and digital optical switches.
A directional coupler switch, as shown in FIG. 3, consists of two coupled semiconductor wave guides. Light is injected into one of two input waveguides. When the signal reaches a region where two waveguides are weakly coupled to one another, light can be transferred from one guide to the other by altering the index of refraction of one of the guide with respect to the other. To construct a 4.times.4 crosspoint switch, four of these couplers are required as shown in FIG. 4.
A Mach-Zehnder interferometer is shown in FIG. 5. This consists of an input waveguide and an output waveguide. Light in the input waveguide is split into two paths associated with respective electrodes. The two paths recombine at the output. Depending on the optical path difference that can be electronically controlled by impressing an electrical signal on one of the electrodes, the interference at the output Y-junction can be destructive or constructive depending on the relative phase of the two paths.
As is the case with directional couplers several of these switches must be cascaded to perform the NXN switching function.
A digital optical switch is shown in FIG. 6. Such switches operate on the "adiabatic change" principle. A beam is split into two optically identical arms with equal intensity in each arm. By influencing electrodes associated with each arm, the effective index in one arm can be reduced sufficiently to transfer all the light in this arm to the other. These switches must also be cascaded to achieve N.times.N switching.
An object of the invention is to alleviate this problem.